The present invention relates to a method and apparatus for arranging the exhaust air and supply air in a drying section of a paper machine or in a corresponding drying section covered by a hood, the drying section having at least one drying group provided with drying cylinders and devices operating by negative or positive pressures.
The drying section of a paper machine or the like utilises devices operating by negative pressure, such as suction rolls, e.g. VacRoll rolls by Metso Paper, Inc., suction boxes and other rolls provided with suction devices, with which a negative pressure is generated at a desired point on a web section between the drying rolls in order to control and stabilise the run of the web. Devices operating by negative pressure means in the following at least all devices in the drying section of the above mentioned type, if nothing else is particularly stated.
On the other hand, the drying section in a paper machine or the like utilises also devices operating by positive pressure, such as blow boxes, for instance UnoRun or SymRunHS boxes by Metso Paper, Inc., which create a negative pressure at a desired point of the web section, the negative pressure attaching the web to the wire and stabilising its run. These devices are called runnability components. Further the drying section utilises other devices operating by a positive pressure, such as blowing hoods, which can direct air blows intensifying the drying directly on the web, or through the wire on the web. Further the drying section uses heating devices and/or devices supplying ventilation air in order to exhaust humid air i.e. from the pockets left between the drying cylinders and the web or the wire, and thus to create advantageous drying conditions. Devices operating by positive pressure means in the following, if nothing else is particularly stated, at least all devices of the above mentioned type in the drying section.
The moisture content which is transferred from the paper web to the air is removed from the drying sections covered by a hood, by removing humid air from the hood space and by substituting the exhaust air by a corresponding volume of drier, so called substitute air. Then the humid exhaust air from the drying section is mainly directly from the air space of the hood, typically via the ceiling of the hood.
On the other hand, devices operating by negative pressure, which remove air from negative pressure locations in order to create a negative pressure, can also be used to exhaust air from the hood. The air removed by the negative pressure devices is typically removed from the drying section together with the air removed through the ceiling of the hood.
Previously it is also known to circulate a part of the air removed from the locations of negative pressure, such as from a suction roll, back to the hood, for instance as blowing air to the blow boxes operating by positive pressure. Then a portion of the humid exhaust air flow from a number of suction rolls is directed to a mixing chamber or the like. In addition to the humid exhaust air from the suction rolls, the mixing chamber receives additional air which can be the above mentioned exhaust air from the air space of the hood, or circulating air, and/or heated substitute air. From the mixing chamber this mixture of circulating air and substitute air is directed to the blow boxes or other devices operating by positive pressure, first to the suction side of their blower and from there further to the device in question.
The air flows exhausted at different places of the hood are generally directed to a common exhaust channel which directs the exhaust air via a heat recovery tower and out from the plant. Generally the drying section is connected to several heat recovery towers, typically two or three towers.
The humid exhaust air from the hood is replaced by substitute air which is typically 50 to 80% drier and which is directed via heat recovery towers to the drying section, i.e. into the devices in the drying section which operate by positive pressure, such as blow boxes or devices which supply heating air or ventilation air. In addition, drier air leaks into the hood i.e. from the machine room.
Thus present drying sections require a large number of air exhausts and blowers for each heat recovery tower in the air system of the hood in order to exhaust humid air from the hood space, to create a negative pressure in the required places, to bring substitute air and to circulate circulating air. Each blower, as well as the numerous channels connected to them, occupy a relatively large space.
The large number of blowers, as well as the large amount of channels connected to them, increases the price of the plant, increases the energy costs and requires much space. The large number of blowers further makes the system complicated and makes it more difficult to control the drying and the air and energy balance.
The object of the present invention is to provide an improvement to the above mentioned disadvantages and to provide an improved method and apparatus for arranging the exhaust air and supply air in a drying section.
Then the object is to provide a method and an apparatus with which the humid air exhaust and the substitute air supply can be realised by fewer blowers than previously.
An object is also to provide in the drying section an apparatus which can be controlled more easily than previously for exhausting humid air and for supplying substitute air and for supplying and exhausting air from the runnability components and suction rolls.
A further object is to provide a method and an apparatus which reduce the circulation of humid air in the hood, as compared to the previously existing circulation.
An object is also to provide a method and an apparatus which reduce one-sided drying, as compared to the previously existing drying.
The apparatus according to the invention is typically suited to be used for arranging exhaust air and supply air in a drying section covered by a hood in a paper machine or the like having a drying group provided with drying cylinders or the like. However, the invention is also suited to be used in such drying sections where the drying group comprises, in addition to or instead of the drying cylinders, other corresponding drying devices known as such, such as suction rolls or the like provided with top blowing hoods. Further, the drying section comprises different devices operating by negative or positive pressure, such as suction rolls, suction boxes, runnability components operating by positive pressure and/or devices which supply heating air and/or ventilation air. Further at least two heat recovery towers are connected to the drying group.
It is typical to a solution according to the invention that it utilises at least two heat recovery towers, through which different air flows are directed, in other words air from different locations and/or intended for different locations.
In a typical solution according to the invention, a heat recovery tower of the first type is dimensioned for a mainly constant volume of exhaust air from devices operating by negative pressure and to supply a mainly constant volume of air to devices operating by positive pressure. The exhaust air and supply air volumes are generally constant, but they can be adjusted when required. Generally 1 to 5 heat recovery towers of this first type, typically 1 to 2 of them, are connected to the drying section. However, there may be a larger number of towers.
In a typical solution according to the invention, the air volumes fed through a heat recovery tower of the second type are typically adjustable. The adjustment of these air volumes controls the volume of humid exhaust air from the hood and the volume of substitute air supply to the hood. Typically there is only one heat recovery tower of this second type, but naturally there may be more when required. The number of the heat recovery towers of the first type is typically equal to or higher than the number, e.g. one, of the last mentioned towers with adjustable air supplies.
The heat recovery towers are typically provided with heat exchangers which can preheat the substitute air supplied to the drying section and/or heat the water used in the processes of the plant. A heat recovery tower means in this invention all such structures with which the heat of the exhaust air flows from the drying section can be recovered for heating different water flows and/or air flows.
A typical apparatus according to the invention comprises, in that part of the drying section where one heat recovery tower of the first type and at least one heat recovery tower of the second type are combined: a first blower for exhausting air from devices operating by negative pressure, via a first heat recovery tower and out from the drying section; a second blower for directing a first substitute air flow via the first heat recovery tower to devices operating by positive pressure, which typically are runnability components, such as blow boxes stabilising the run of the web; a third blower for directing a second substitute air flow via the second heat recovery tower to other different devices operating by positive pressure, which typically are devices supplying heating and ventilation air; and a fourth blower for exhausting air from the hood, typically from the ceiling of the hood, and for directing it via the second heat recovery tower and out from the drying section.
In the solution according to the invention a part of the exhaust air from the hood can be circulated, as mixed with substitute air, by the second blower back to the hood, to some devices operating by positive pressure.
As previously mentioned, a number of heat recovery towers representing the first and second types can be connected to the drying section. Then also corresponding additional blowers are arranged in the drying section, in other words blowers for each heat recovery tower in accordance with the above presentation.
In this invention a blower means all conventional blowers or corresponding devices, which are suitable for transporting the air flows in the drying section.